Photosurface of a panchromatic type and method of preparing same



June 29, 1954 R JOHNSON 2,682,479

PHOTOSURFACE OF A PANCHROMATIC TYPE AND METHOD OF PREPARING SAME FiledMarch 3, 1949 2 Sheets-Sheet l .IIIIIIIIIIIIIIIIIIIA Tube (00 NVENTORRALPH E. JOHNSQN @050 3020 we BY TYPE OFPHUTQSUEFACE ear/a 0F 5/11 51?TOBISMI/I'H //v SURMGE ATTORNEY Patented June 29, 1954 t PHOTOSURFACE OFA TYPE AND METHO SAME Ralph Eston Johnson, Lancaster,

PAN CHROMATIC D 0F PREPARING Pa., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application March 3, 1949, Serial No.79,328 11 Claims. (0]. 11733.23)

This invention relates to the method of making a photoemissive surfaceof general application, but having a particular use in television cameratubes.

One type of television camera pickup tube utilizing the novelphotoemissive surface formed according to my invention, is that havingan image section consisting of a photocathode electrode formed on theend plate of the tube envelope and a thin glass'target electrode spacedfrom the photocathode. An optical image focussed upon the photocathodereleases a photoemission which is accelerated and focussed so that itstrikes the thin glass target electrode with an energy sufficient tocause a secondary emission from the glass surface greater than unity.The secondary emission leaves a positive charge pattern on the glasstarget surface corresponding to the optical image focussed upon thephotocathode. The opposite side of the glass target is scanned by anelectron beam which is caused to approach the target at close to 'zerovelocity. Due to the extreme thinness of the glass target, theelectrostatic image on the one side of the target sets up the same imagepotential on the scanned side of the target. Electrons from the scanningcathode ray beam will be drawn to the positive areas of the targetsurface and will be deposited on the target to neutralize the positivepotential pattern on the glass. The deposited beam electrons dischargethe target surface to cathode potential at which point the remainder ofthe beam is reflected back along the path and is collected to form videosignal of the tube.

In the type of television camera pickup tube described above, twodifferent photoemissive surfaces have found application. Onephotosurface is formed from a silver-antimony alloy which is sensitizedwith caesium. This photosurface has low response to infra-red, adesirable characteristic, and the colors of the visible spectrum arereproduced in a reasonably satisfactory manner under controlledconditions of illumination and lighting. However, this silver-antimonyphotosurface has a relatively low sensitivity of approximately 6microamperes per lumen. A television 'camera pickup tube utilizing thisphotosurface is used mainly with controlled lighting or where intensityof illumination is sufliciently high. An-

other type of photosurface is formed from silveri all light levelsranging from bright sunlight to that of deep shadow. However, thecaesiated silver-silver oxide photosurface has too great an infra-redresponse, so that colors are not reproduced faithfully, i. e. as thehuman eye sees.

them, giving an unnatural appearance to the television picture. If oneattempts to use filters to correct the response, the sensitivity isreduced to that of the silver-antimony photosurface or less.

The silver-antimony photosurface, as suggested above, is better adaptedto controlled lighting conditions, while the caesiated silver oxidephotosurface will be sufliciently sensitive for use in television tubesunder natural lighting conditions of outdoors. Because of this, it isnecessary to use two different camera tubes in order to televise thewhole range of lighting, conditions which may be required.

tures from outdoor or poorly lighted sets do not match those transmittedfrom the studio, where controlled lighting is possible, resulting in anadditional objection from an artistic point of View. Therefore, a verydesirable feature in a photosensitive surface used specifically fortelevision camera pickup tubes would be to have a color spectralresponse matching that of the human eye and also having a maximumsensitivity permitting such a camera tube to be used under allconditions of lighting.

It is an object of my invention to provide a photoemissive surface ofgreat sensitivity.

It is another object of my invention to provide an improvedphotoemissive surface having high spectral response throughout thevisible region.

It is a further object ofmy invention to pro-' vide an improvedphotoemissive surface having spectral response similar to visualresponse.

It is a further object of my invention to provide an improvedphotoemissive surface with negligible response outside the visibleregion.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but the inven-Furthermore, when using two difierent camera tubes, the piction itselfwill best be understood by reference to the following description takenin connection with the accompanying drawing, in which:

Figure l discloses a type of television camera pickup tube utilizing aphotosurface made according to my invention.

Figure 2 is an enlarged view of the image s ec tion end of the tubeshown in Figure 1.

Figure 3 is a view of the image section shown in Figure 2 and viewed inthe direction of the arrows 33 of Figure 2.

Figure 4" is a' graph showing relative responses of variousphotosurfaces as compared with that of the novel photosurface madeaccording to my invention.

Figure 5 is a graphical representation of the photosensitivity of aseries of photosurfaces made according to my invention.

Figure 6 is an enlarged sectional view of the photocathode of the tubeof Figure ii in accordance with the invention.

The particular photosensitive layer which is the subject of my inventionmay be applied to any desired supporting surface which may be eitheropaque or transparent. The specific use of my new photosurfacei's intelevision camera pickup tubes either of the image orthicon type inwhich a transparent photocatho'de is used; or in an iconoscope in whichthe photocathode is opaque and the scene is focussed upon the same sideof the photocathode. electrode as is scanned by the electron beam.

In Figure l is disclosed a television camera pickup tube utilizing aphotcemissive surface made according to my invention; In operating thetube of Figure 1, a scene which is to be televised is focussed' upon theend plate 30 of the tube envelope i0. Photoelectrons are released from asemi-transparent photoemissive surface 3|, formed on the inner surfaceof .the, face plate 3i The photoelectrons are acceleratedby a ringelectrode 32 so that they will strike a thin glass target 24 withsufficient energy to cause a secondary emission-greater. than unityfromthe surface of glass 24. surrounding the envelope. ill provides afieldco.- axial with the tube envelope H! for focussing the photoemissionupon the glass target 24. A fine mesh screen 34-isclosely spaced fromthe surface of target 24 and. functions as a collector 5 electrode ofthe secondary emission, The secondary emission from the surface of theglass target 24 leavesa positive charge pattern on the glass targetsurface corresponding to. the optical image focused upon thephotocathode3i... The opposite side of the glasstarget 24 is scanned by an electronbeam formed by an. electron gun structure represented at. 16. Theelectron beamv is caused to approach. the opposite surface of target 24at close to zero velocity through the use of a decelerating electrode22.. Due to the extreme thinness of the. glass target 24-,theelectrostatic emission on the. photocathode side of the target 24sets up the. same image potentialon the side-of the-target 24 scanned bythe electron beam. Electrons from the cathode ray beam will be drawn tothe. positive. areasof thetarget surface and will be deposited on thevtarget to neutralize the positive potential pattern on the glass 24.Thedeposited beam. electrons discharge the surface of glass target 24 tocathode potential at which point the remainder of the beam i reflectedback along toward the gun structure Hi. This return beam is amplified.by passing it through a multiplier section 28 in which it is An.electro-magnetic coil 36 4 finally collected to form the video signal ofthe tube.

As shown in Figure 2, the photoemissive surface 3! of the tube of Figure1 is connected to an external circuit through a silver stripe 38 incontact with the layer 3| and a lead 39 connected between stripe 38 anda stem pin 4!.

To form my improved photosurface, an alloy of pure silver and purebismuth is made up by weight in the ratio of 9 parts of silver to 1 partof bismuth. The mixtureis heated until melted and is continuouslystirred until the metals are well mixed together. Upon cooling, thematerial. forms an alloy mass. The silver-bismuth alloy may be put downupon the supporting target surface in. any desired manner. Asatisfactory method. is that of evaporating the alloy from heatedfilaments, made from a molybdenum-tungsten alloy. For example, as shownin Figurev 3, small.- quantities 40 of the silver bismuth alloy maybeattached to an evaporator filament 42 by melting the si1ver-bismuthalloy and applying it to-filament 42 at temperatures below evaporation.The melted silver-bismuth alloy tends to wet the filament wire 42 andwill cling to the wire as a globule 40 and be firmly attached-uponcooling.

During the assembling of the tube of Figure l, filament 42 with thesilver-bismuth alloy globules is mountedwithinthe cylindrical electrode26- and beyond the peripheral edge of the mesh 34. as shown in Figure.3. One end of the filament 42 is connected toa lead 46 (Figures 2 and 3)connected to. a stem pin 48 passing through the envelopewall as shown.The other end of the filament 42 is connected at 44 to electrode 26which can be connected externally of the envelope through. lead 5B.andstem pin 52. In this manner, filament 42. is mounted within theenvelope lfl, close. to the face plate 30, upon which the photosurface31. is to be formed. Two or more evaporators, may. be arrangedsymmetrically about. the axis of the supporting surface 30,.

in place of the singlefilament 42.

The tube H3, in which the photosurface is to be formed, is firstprocessed and evacuated, andv thenbaked between. 3'75-400 C. for onehour to remove occluded. gasses from the envelope. The metal electrodeswithin the tube are then heated and degassed, as is well known in theart. Also, if the tube has a thermionic cathode, this is activated. atthis point. processing of the tube, an electric current is passedthrough the evaporator filament 42 by the use of lead pins 48 and 52 toheat the filament to a temperature sufficiently high for evaporating thesilver-bismuth alloy layer 3 l In formingthe photocathode surface on thetransparent face plate 30, the evaporator 42 is operated to depositsufficient silver-bismuth alloy material to changethe light transmissionthrough plate 30 to 38% of the normal light transmission throughsupporting plate 39 before any film is deposited; This can be easilydetermined by passing a beam. of light through the face plate.

30. and causing it to impinge upon a photocell connected to-appropriatecircuits. As the silverbismuth alloy is deposited upon the surface, thecurrent flowing through the photocell is continuously indicated and theevaporation is stopped when the current has dropped to 38% of itsinitial reading.

After the silver-bismuth alloy is deposited on the supporting surface30, it is oxidized" until the light transmission through the transparentsup- After this normal port 30 is raised to 50% of the normal or initialtransmission of light prior to the putting down of the alloy layer. Theoxidation step may be performed in any manner. One successful method isto admit oxygen into the tube envelope to a pressure between 300 and 500microns. The silver-bismuth alloy film is then oxidized by a glowdischarge produced within the tube and adjacent the supporting surface.The discharge is produced in a manner described in U. S. Patent2,020,305 to Essig, in which a, portable electrode is used and which isconnected to one terminal of a radio frequency oscillator circuit, theother terminal of which is grounded. The portable elec trode is slowlypassed over the silver-bismuth surface several times until the lighttransmission becomes as stated above. During this step, a blue dischargeis necessary to provide the proper conditions of oxidation. Afteroxidation of the silver alloy film, a layer of caesium metal isdeposited upon the oxidized alloy film to provide photosensitivity ofthe layer. The step of depositing caesium may be done in any well knownmanner, and as described below.

To provide a source of caesium to caesiate the silver-bismuthphotosurface 31 in the tube of Figures 1, 2 and 3, a mixture of caesiumchromate and silicon, as a reducing agent, is placed into a pair ofchannels 54, which are mounted as is shown in Figures 2 and 3 on theouter surface of electrode 26. The channels are metallic and are formedas hollow troughs to hold the mixture of caesium chromate and siliconwith suitable binding material to keep the material securely in thechannel. One end of each channel 54 is attached as shown at 5'8 to thewall of the electrode 26, customarily by welding. The two metallic endsof each channel have been previously formed as extensions 51 so that thecaesium activator mixture is confined to the hollow trough of thechannel and does not interfere with this latter welding operation. Theother end of each is connected to a lead member 58 which may be anextension of the stem pin 60.

After the previously described step of oxidizing the silver-bismuthsurface 3|, an electric current is passed through the stem lead 52 and60 to heat the channel 54 to a sufficiently high temperature to cause achemical reaction in the chromiumsilicon mixture which will release purecaesium as a vapor in the photocathode end of the evacuated envelope l0.During this evaporation of the caesium metal, the tube I0 is maintainedbetween 150 C. and 200 C. by being placed within an oven. Thistemperature of the tube maintains the caesium metal in vapor form andallows it to pass over onto the oxidized silver-bismuth alloy surfaceand combine uniformly with the semitransparent metallic layer. Thecaesium metal vapor is caused to be deposited upon the silverbismuthalloy surface 3! until the alloy surface provides maximumphoto-sensitivity. This may be determined by illuminating thephotosurface with light during the evaporation of the caesium metal. Anammeter in the photocathode circuit of the tube is watched until itregisters a maximum photoemission. When this occurs the current throughthe channel is stopped by turning off the applied voltage and evolutionof caesium ceases. This method is sufiiciently precise so that verylittle caesium beyond that necessary to properly sensitize thephotosurface is introduced into the tube. Excess caesium, if any, willharmlessly be absorbed by the other parts or pumped out through theexhaust tubulation as the tube is still sealed to an exhaust'systemfor-this operation. The above method of putting down a sensitizedsilver-bismuth alloy photosurface has been described mainly inconnection with the formation of a photocathode on a transparentsupport. However, it is also possible to form a photosurface upon anopaque support member by a similar method.

The above method of making a photosensitive surface was described asusing a silver-bismuth alloy consisting by weight of 90% of silver, and10% bismuth. Photosurfaces formed in the manner described above andformed by an alloy having a silver to bismuth ratio of 90 to 10, havebeen found to have a sensitivity varying between 36 microamperes perlumen and 72 microamperes per lumen and have averaged around 53microamperes per lumen. It is not necessary, in making a photoemissivesurface according to my invention, to be limited to the silverto bismuthratio of 90 to 10, as good photoemissive surfaces have been made withsilver to bismuth ratios by Weight of 92517.5, 93.5:6.5 and of 95 to 5.Photoemissive surfaces made in the manner described above and laid downfrom a silver bismuth alloy having a ratio of the weight of silver tobismuth equal to 93.5:6.5 have been consistently good and have averagedaround 45 microamperes per lumen. A series of photoemissive surfacesmade with a silver to bismuth Weight ratio of 95:5 have shown a range ofsensitivities between 36 micro amperes per lumen to 76 microamperes perlumen, averaging around 52 microamperes per lumen. Above this ratio ofsilver to bismuth, the sensitivity of the photoemissive surface appearsto drop off. Tubes made with a silver bismuth photosurface having aratio of weight of silver to bismuth of 97.5:2.5 show an average ofaround 10 microamperes per lumen, while photoemissive surfaces madeentirely of silver with no bismuth alloyed therewith, and according tothe method described above, have relatively poor photosensitivity ofaround 5 microamperes per lumen or less, and/or have undesirablespectral response characteristics as previously mentioned. Furthermore,photoemissive surfaces made with a silver to bismuth weight ratio of 80to 20, according to the methoddescribed above, also have relatively poorsensitivity, which ranges from 2 microamperes per lumen to 5microamperes per lumen. Photoemissivesurfaces made with a lowerproportion of silver than 80% have consistently shown poor sensitivity,for example, photosurfaces formed from an alloy having a silver tobismuth weight ratio of 50:50 have sensitivities ranging between 2microamperes and 5 microamperes per lumen. From experimental data, itappears that there is an optimum range for the silver to bismuth weightratio lying within the values of 80 to 20 and 98 to 2. Figure 5 is arough representation of the results obtained from a series ofphotoemissive surfaces made according to the method described and fromvarying proportions of silver to bismuth in the metal alloy used for thephotosurface. The curve of this figure represents the values of thevarious photosurfaces described above. It is easily seen from the figurethat maximum photoemission is obtained from those photosurfaces formedfrom alloys having ratios of the weight of silver to the weight ofbismuth lying between :15 and :5 while auseful range is obtained fromphotosurfaces having silver to bismuth Weight ratios lying in the rangefrom 80:20 and 98:2. Furthermore, as described above, it has :also beendetermined -;that "some photoemissive"sensitivityris .--obtained from-practically the whole range of values for the weight ratio of silverIUbiSmHtIIffI'OMEOZZ JU to 100:0.

The. novel photosensitive surface described above is one -which.tprovide's arcolor "response as shown-in curve'l2 ofFigureA,:which isvery similar to the response of the human eye, as shown by curve 10.Theesimilarityrof :the two curves is and 12 iseven more'noticeable whenconsidered 'with' 'curves 14 and '1 l6 1 which respectively representspectral "responses :of two above-described; caesiatedzsilver silver.oxide photosurfaces and: also withrcurvef'l8 representing the spectralresponse "of ncaesiated; silver-antimony alloy photosurface; Although'the -"caesiated silver-silveroxideftyperof:photosurface has arelativelyhigh .sensitivityzof around 20 microamperes per lumen;tit"canbeseen from the curves 14 and 16 of Figure 4;"that the-color response ofthis type of photosurface extends 'far beyond the red region:oftheinvisible spectrumand a great amount of .its' sensitivity isderived from its response to infra-red energy. This energy is presentinnearlyall. practical scene illuminations to a:greater-ortlesseriextentbut is not'received as visible energy by the human eye.

Camera tubes using these caesiated silversilver oxide photosurfaces',therefore, introduce what is known in the industry: as color masking.Thatis; they respond and give indication of light where --.the eye seesnone, masking out the truev color gradations seen by theeye. Thisresults in the transmission of very unnaturallooking pictures; which 1is undesirable in television practice. Furthermore, as may be seen fromcurves and 16, it has been very diflicult to produce similar responsecharacteristics among individual tubes employing-this surface. That is,the spectral response-of the silver-silver oxide surface is subjecttoappreciable variations due to factors vthatare not under control evenby those skilled .in' the art; Consequently, in addition to everysurface exhibiting response characteristics unsuited totelevision,individual tubes vary in their relative response to red and infraredenergy so that. it is very diflicult to get tubes with this type surfaceto match each other.

The spectral response for a caesiated silverantimony alloy photosurfaceis illustrated by curve 18. This response-can be duplicated fairly wellfrom tube to tube and, although it possesses extremely high sensitivityto blue light compared to the eye response; the tonal response to coloris not'too objectionable because it is essentially confined to thevisible spectrum. However, the-sensitivity'of such surfaces is too lowto be used universally, so that where light levels are inadequate, acamera tube employing the silver-silver oxide surface must be employed.

From the curves shown-in Figure 4, it can be easily, seen that'ithenovelsilver-bismuth photosurface .follows'the normal eye response in allwave lengths much more closely than the other types of the photosurfaceswhich .are in use in television camera:pickup tubes.

Furthermore: this sensitized silver-bismuth photosurface hasexceptionalsensitivity, which on an average 'hig'herthan" any other photosensitivematerial used in television camera tubes.

As described above; this silver-bismuth alloy photosurface-haswasensitivity averaging above 50 microamperes per lumen. Due to this greatsensitivity,.televisionucamera tubes using this 8 silver-bismuth alloyphotosurface are capable of televisingiscenes under any lightingconditions encountered inrazxpractical :sense. Furthermore, thesensitivity of thisphotosurface is achieved while atthe .same timeapproaching the eye sensitivity curve more closely than any of the othersurfaces knownto the art. Because of the higher sensitivity and improvedspectral response, filters maybeused'under certain conditions toapproach' the eye response curve even more closely withoutwseriouslyaffecting. the performance of the tube. Because of these very decidedadvantages, the silver-bismuth photosurface has found extensive use intelevision cameras under all conditions, indoors,'out-of-doors, andunder television studio conditions. Television pickup tubes using thissilver=bismuth photosurface produce better quality pictures with ahigher degree of uniformity than has been achieved previously.

While certain specific embodiments have been illustrated anddescribed,it'will be understood that various changes andmodifications may be madetherein -withoutdeparting from the spirit and scope of the invention.:.

What -I claim is:

l. A photosensitive electrode comprising, a support, a layer ofsilver=bismuth alloy on the surface of said. support, and a deposit ofalkali metal on said alloy layer.

2. A photosensitive electrode comprising a support, a layer of oxidizedsilver-bismuth alloy on a surfaceof said support, and a deposit ofalkali metal. on said'oxidized alloy layer.

3. A photosensitiveelectrode comprising a support, a layer of oxidizedsilver-bismuth alloy on a surface "of said support, and a deposit ofcaesium onthe surfaceof saidalloy layer.

4. A photosensitive electrode comprising a support, a film on onesurface-of said support including an alloy of silverand bismuth combinedwith oxygen and caesium.

5. A photosensitiveelectrode comprising a film on a surface-of saidsupport'including an alloy of silver and bismuth combined with oxygenand caesium, theratio of the weight of silver to the weight of bismuthin said alloy being between 50:50 and 98:2.

6. A. photosensitive electrode comprising a support;v a film onaasurface of :said support including an alloy of silver and bismuthcombined with oxygemand caesium; the ratioof the weight of silver to theweight of bismuth in said alloy being between :20 and98:2.

'7. A photosensitive electrode comprising a support, a film on asurfaceof said support including an alloy'of silver and bismuth combined withoxygenand caesium, the ratio of the weight of silver to the weight ofbismuth in said alloy being 10.

8. A photosensitive"electrode comprising a support, a film on a surfaceof said support including an alloy of silver and vbismuthcombined withoxygen and caesium, the ratio of the weight of silver to the weight ofbismuth in said alloy being :5.

9. The method vof making a photosensitive electrode, said methodcomprising the steps of, evaporating upon a support having a normaltransparency a layer of silver-bismuth alloy until the lighttransmission throughsaid support is reduced'to approximately 38 per centof normal transparency, oxidizing said silver-bismuth alloy layer untillight transmission through said sup-. port is 50% of normaltransparency, sensitizing said oxidized silver-bismuth alloy surface bydepositing caesium vapor upon said alloy layer.

10. A photosensitive electrode comprising a support, a film on onesurface of said support including an alloy of silver and bismuthcombined with oxygen and caesium, the ratio of the weight of silver tothe weight of bismuth in said alloy being between the values of 85:15and 95:5.

11. A photosensitive electrode comprising a support, a film on onesurface of said support ineluding, an alloy of silver and bismuthcombined with oxygen and caesium, the ratio of the weight of silver tothe weight of bismuth in said alloy being 92.2:7.5.

References Cited in the file of this patent UNITED STATES PATENTS Number5 1,738,957 1,894,946 2,123,024 2,244,720 2,254,073 10 2,285,0582,285,062 2,297,467 2,401,737 2,404,803 15 2,413,442

Name Date Metcalf Dec. 10, 1929 Espe Jan. 24, 1933 Piore et a1. July 5,1938 Massa et al. June 10, 1941 Klatsow Aug. 26, 1941 Samson June 2,1942 Sommer June 2, 1942 Gorlich Sept. 29, 1942 Janes June 11, 1946Stafiord July 30, 1946 Found Dec. 31, 1946

1. A PHOTOSENSITIVE ELECTRODE COMPRISING, A SUPPORT, A LAYER OFSILVER-BISMUTH ALLOY ON THE SURFACE OF SAID SUPPORT, AND A DEPOSIT OFALKALI METAL ON SAID ALLOY LAYER.